1. Field of the Invention
The present invention relates to an electron beam writing method and a fine pattern writing system for writing a fine pattern according to a desired uneven pattern when manufacturing an imprint mold, magnetic transfer master substrate, or the like for a high density magnetic recording medium, such as a discrete track medium, bit pattern medium, or the like.
The invention also relates to a method for manufacturing an uneven pattern carrying substrate, including an imprint mold, magnetic transfer master substrate or the like, having an uneven pattern surface formed through a writing step performed by the electron beam writing method described above. The invention further relates to a method for manufacturing a magnetic disk medium having an uneven pattern transferred thereto from the imprint mold of uneven pattern carrying substrate, and a method for manufacturing a magnetic disk medium having a magnetized pattern transferred thereto from the magnetic transfer master substrate.
2. Description of the Related Art
Generally, information patterns, such as servo patterns and the like are formed on current magnetic disk media. In view of the demand of higher recording density, a discrete track medium (DTM) in which magnetic interference between adjacent data tracks is reduced by separating the tracks with a groove pattern (guard band) has been attracting wide attention. A bit pattern medium (BPM) proposed for achieving still higher density is a medium in which magnetic substances forming single magnetic domains (single-domain particles) are physically isolated and disposed regularly, and one bit is recorded in one particle.
Heretofore, fine patterns, such as servo patterns and the like, have been formed on magnetic media by uneven patterns, magnetic patterns, or the like and an electron beam writing method for patterning a predetermined fine pattern on a master of a magnetic transfer master substrate or the like has been proposed. In the electron beam writing method, a pattern is written on a substrate applied with a resist by emitting thereon an electron beam corresponding to the shape of the pattern while rotating the substrate as described, for example, in Japanese Unexamined Patent Publication No. 2000-207738 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2003-248981 (Patent Document 2).
When writing fine patterns by the electron beam writing method described above, it is difficult to accurately write elements constituting each pattern on the entire surface from the inner to outer circumferential tracks.
That is, according to Patent Document 1 and Patent Document 2, inner circumferential track writing and outer circumferential track writing are performed by rotating the substrate at the same rotational speed. Consequently, the amount of radiation exposure differs for a resist having the same sensitivity between the inner circumferential track and outer circumferential track because the moving speed of the substrate relative to the electron beam is decreased in the inner circumferential track and increased in the outer circumferential track, thereby posing a problem that uniform beam exposure can not be realized.
It is difficult to control the electron gun so that the output power of electron beam is changed slightly because of structural reasons thereof. Therefore, it is necessary to control the moving speed of the substrate or beam deflection speed in the electron beam exposure according to the radius of the substrate at the electron beam emission position.
In particular, in Patent Document 1 and Patent Document 2, the writing range in a circumferential direction (rotational direction) is defined by ON/OFF control of the electron beam according to the rotational position of the substrate, that is, the beam emission position, so that it is generally the case that the substrate is rotated at a defined constant speed (number of rotations) regardless of the writing position as described above, and the ON/OFF control based on the write pattern data is performed according to an encoder pulse signal generated in association with the rotation of the substrate.
This method has an advantage that the control is simplified since the substrate is always rotated at a constant speed, that is, the rotation time of the substrate for one rotation is always the same, and both inner and outer circumferential portions can be controlled by the timing according to one type of encoder pulse. But the method has a problem that it is difficult to realize beam writing with a constant amount of radiation exposure (dose amount).
That is, when the substrate is rotated at a constant speed, that is, when the rotation time of the substrate for one rotation is the same between inner and outer circumferential writing, the relative moving speed between the emission beam and substrate surface (writing surface) is increased, causing insufficient dose amount unless the beam intensity is increased. On the other hand, the relative speed is decreased in the inner circumferential writing, causing a problem of excessive amount of dose.
Various methods for dose amount control are proposed, but as far as ON/OFF control is concerned, the dose amount control is fundamentally time control based on encoder pulse. Therefore, it is difficult to ensure favorable writing results for a material requiring strict dose amount control.
In these respects, the present invention attempts to provide an electron beam writing method capable of writing a high precision pattern by enabling accurate ON/OFF control of beam emission in a circumferential direction of the substrate for the writing of complicated patterns, such as servo patterns, over the entire surface of the substrate.
That is, the present invention performs rotation control for controlling the rotation speed of a rotation stage on which a substrate is placed so as to be decreased inversely proportional to the radius of writing position of the electron beam in ON/OFF control, thereby keeping the same rotational linear velocity for inner track writing and outer track writing to maintain the same amount of radiation exposure per unit area and to allow pattern writing with the same quality for a resist having the same sensitivity.
The change in the rotational speed in the writing between the inner and outer circumferential tracks results in that the rotation time of the rotation stage for one rotation differ from each other. In the mean time, in order to allow identical signals to be read out from inner and outer circumferential tracks of an actually used magnetic disk medium in one rotation, it is necessary to accurately form fine pattern elements according to the radial position such that the lengths thereof in a track direction become short in the inner circumferential track and long in the outer circumferential track In order to accurately control the scanning of the electron beam writing, it is necessary to control the beam scanning accurately in synchronization with the rotational movement of the rotation stage. Generally, the position and timing control is based on a clock signal generated at a constant time interval.
Where the rotation time of the rotation stage for one rotation differs in the writing between inner and outer circumferential tracks, it is necessary to finely control the length of a writing pattern in a track direction by counting different number of clocks for different tracks. In particular, for a track where the time corresponding to the writing length does not correspond to an integer multiple of the clock signal, having a fraction, the electron beam emission control is performed without regarding the fraction, causing degraded accuracy of written pattern and complicated write control. Therefore, it is also desirable to eliminate these problems.
In view of the circumstances described above, it is an object of the present invention to provide an electron beam writing method capable of writing a fine pattern, to be formed on a magnetic disk medium, over the entire surface of a substrate in a predetermined manner and highly accurately with a uniform amount of radiation exposure over the entire substrate, and a fine pattern writing system for implementing the electron beam writing method.
It is a further object of the present invention to provide a method for manufacturing an uneven pattern carrying substrate, such as an imprint mold or a magnetic transfer master substrate, having a fine pattern accurately written by an electron beam, and a method for manufacturing a magnetic disk medium using the uneven pattern carrying substrate in which an uneven pattern or a magnetic pattern is transferred from the uneven pattern carrying substrate to the magnetic disk medium.